Cryopreservation of human embryos (Cryopreservation=Freezing)
After the process of ovarian stimulation, egg fertilization and embryo culture, embryos can be frozen for the future use. In the case of the freeze-all protocol, all embryos will be frozen at five or six days at which stage they are called blastocyst. In the case of fresh transfer, the surplus embryo can also be frozen for future use. Usually, embryos can be frozen at any stage starting from one cell (two pronuclear stage) until the blastocyst stage. From our experience of more than ten years in embryo cryopreservation the stages at which we prefer to do freezing are at the one cell and blastocyst stages because the survival rate is extraordinarily high at both these stages.
Methods for embryo freezing nowadays are slow programmable freezing and vitrification (the Latin word for turning fluid into a glass-like solid). Both methods are comparable in terms of survival and success in both one-cell and blastocyst stage embryos. We have much experience of vitrification in both stages, with a very high survival rate of more than 95%.
Vitrification is the ultra-rapid freezing method, which is much faster than traditional slow programmable freezing. The difference between this process and slow freezing is the very high concentration of freezing medium which is enough to turn the fluid into a glass-like solid without ice crystals forming inside the embryonic cells when they come into contact with liquid nitrogen. Due to the development of new combinations of cryoprotectants, the survival rate of vitrified embryos is very high compared with the previous method. Due to the fact that freezing has no ice crystal formation inside the cell and the development of new combinations of cryoprotectants, the survival and implantation rate after thawing by vitrification technique is very high compared with the traditional method. This is one of the new technologies that can improve the chances of IVF success worldwide.
The embryos can be frozen at any stage of development, however, the best stage for freezing in terms of survival and chances of success is the blastocyst stage. Many IVF clinics all over the world are currently changing the stage of freezing to the blastocyst stage. We have more than 13 years’ experience of blastocyst culture and freezing and now we prefer this stage for freezing and transfer only. Furthermore, the freeze-all protocol and elective frozen embryo transfer are becoming much more widely used techniques today.